Emergence of glassy-like dynamics in an orientationally ordered phase

Abstract

The dynamics of a simple rigid pseudoglobular molecule (2-adamantanone) has been studied by means of dielectric spectroscopy and examined under the constraints imposed by the space group of the crystal structure determined by x-ray powder diffraction. The low-temperature monoclinic structure of 2-adamantanone, with one molecule per asymmetric unit (${Z}^{\ensuremath{'}}=1$), displays a statistical intrinsic disorder, concerning the site occupancy of the oxygen atom along three different sites. Such a physically identifiable disorder gives rise to large-angle molecular rotations which inherently lead to time-average fluctuations of the molecular dipole, thus contributing to the dielectric susceptibility. The dielectric spectra for the low-temperature ``ordered'' phase displays a universal feature of glassy-like materials, i.e., coexistence of \ensuremath{\alpha}- and \ensuremath{\beta}-relaxation processes. The former is clearly identified with the strongly restricted reorientational motions within the long-range ``ordered'' crystalline lattice. The latter, never observed before in fully translationally and highly orientationally ordered phases, displays all the properties of an original Johari-Goldstein \ensuremath{\beta}-relaxation, in spite of the strong character of this glass-like phase. These findings can be explained according to the coupling model, applied to such ``ordered'' phases.